My shop made solution was to drill an oversized hole in the knob's base into which I glued a metal tube that I modified to have a nice friction fit with the wide part of the shaft. I then drilled and tapped holes for set screws into the lower sides of the knob to secure the knob to the shaft. With the knob installed the set screws are hidden by the leather boot. This works well for me and I'm well pleased with the result.
When and if I turn another knob for my Boxster I think I'd forgo the metal insert and drill a series of 7mm holes to create a slot for the rectangular shaft.
Of course the quick and easy solution would be to buy an aftermarket knob that was made to fit our cars. These are available from many sources including Pelican Parts and eBay.
This is the walnut knob I turned shortly after I bought the car.
cheers - dj